Hematopoietic cell-specific Lyn substrate associated protein-1 (HAX-1) was discovered in 1997 and suggested to play an anti-apoptotic role in B-cells. Although HAX-1 is ubiquitously expressed in all tissues, its role in the heart remains virtually unknown. Interestingly, recent studies demonstrate that HAX-1 interacts with two main mediators of sarcoplasmic reticulum (SR) calcium uptake, phospholamban (PLN) and sarco(endo)plasmic reticulum calcium ATPase (SERCA), suggesting its role in regulating cardiomyocyte calcium homeostasis. Moreover, HAX-1 level is reduced in cardiac ischemia/reperfusion injury, implicating the participation of HAX-1 in this disease. Thus, to elucidate the role of HAX-1 in the regulation of calcium kinetics and cardioprotection, HAX-1 over-expression and HAX-1 heterozygous deficient mouse models were used in the present study for the examination of alterations in basal contractile performance and recovery after ischemia/reperfusion injury by HAX-1.
First of all, this dissertation reveals that HAX-1 is cardioprotective against ischemia/reperfusion. 2.5 fold over-expression of HAX-1 reduced infarct size and improved recovery after injury, which was ascribed to reduced apoptosis and necrosis. At the endoplasmic reticulum (ER) level, HAX-1 specifically inhibited the inositol requiring enzyme-1 (IRE-1) ER stress signaling pathway and prevented the subsequent apoptosis induction. This protection was lost in HAX-1 heterozygous knockout hearts (36% expression), which led to increased infarct size. At the mitochondria level, HAX-1 protected mitochondrial membrane integrity by reducing cyclophilin-D (Cyp-D) expression and thus limiting the opening of mitochondria permeability transition pore (mPTP). This conferred protection against the activation of apoptosis and necrosis. More importantly, these beneficial effects were at least partially mediated through the interaction with heat shock protein 90 (Hsp90). HAX-1 appeared to compete for Hsp90 binding with IRE-1, causing attenuation of IRE-1 activation. On the other hand, HAX-1 facilitated the recruitment of Cyp-D to the mitochondrial Hsp90 chaperone complex, which might promote Cyp-D degradation.
Moreover, this dissertation demonstrates the novel role of HAX-1 in calcium handling and contractility regulation. Over-expression of HAX-1 resulted in suppressed SR calcium re-uptake activity, reduced SR calcium content and depressed calcium kinetics, which were translated into reduced cardiomyocyte contractility. On the other hand, HAX-1 heterozygous deficiency led to enhanced calcium kinetics, which improved cardiomyocyte contractility. The attenuation of calcium kinetics by HAX-1 was mediated through PLN, as increasing HAX-1 promoted the formation of PLN monomer, the active SERCA inhibitor. Removal of PLN function, by either gene ablation or phosphorylation, abolished the effect of HAX-1. Additionally, Hsp90 is implicated in HAX-1 regulation, as inhibition of Hsp90 activity increased calcium kinetics.
Lastly, the present study suggests that HAX-1 is protective in chronic cardiac stress. Eight weeks after myocardial infarction, induced by an in vivo ischemia/reperfusion procedure, HAX-1 transgenic mice showed reduced cardiac hypertrophy, compared to WT littermates. Moreover, decline in ejection fraction was prevented by HAX-1 over-expression, supporting a protective role of HAX-1 in the progression of heart failure.
Collectively, this dissertation demonstrates that HAX-1 can: a) inhibit apoptosis and necrosis at both ER/SR and mitochondria levels through the formation of HAX-1/Hsp90 complex; and b) regulate SR calcium cycling by modulating PLN activity, which can in turn regulate cardiomyocyte contractility.